The manufacture of integrated circuits involves placing controlled amounts of impurities into tiny regions located on a semiconductor substrate that are subsequently interconnected. This is generally accomplished by a lithographic process that includes applying a thin film of photoresist onto a substrate, usually silicon or silicon dioxide, and exposing the photoresist to radiation such as ultraviolet light, x-rays, an electron beam, an ion beam or the like. An obstacle to the radiation, such as a mask or recticle, is placed between the source of radiation and the photoresist layer to selectively expose the photoresist layer with a desired pattern. The areas exposed to radiation are either soluble or insoluble in a solvent known as a developer. If the exposed photoresist is soluble a positive image is produced and the photoresist is termed a positive resist whereas if the exposed photoresist is insoluble in the developer it produces a negative image in the photoresist layer and the photoresist is termed a negative resist.
Subsequent to development, the areas of the substrate not covered by the photoresist are subject to further processing steps while the areas of the substrate still covered by the photoresist are protected from these process steps by the photoresist. This process is repeated until the desired integrated circuit is completed in a manner well known in the art.
As integrated circuits become more complex it is desirable that increasing numbers of active and passive electrical elements be included on a single chip. This is best accomplished by decreasing the physical size of the active and passive elements rather than by increasing the physical size of the chip. Smaller geometries however are limited by the resolution capabilities of the lithographic process and include hardware limitations such as the mechanical stability of the system, the optical characteristics of the lens, light source, etc. Optical and process characteristics of the photoresist also affect the best resolution that can be obtained with any lithographical system.
Heretofore in the prior art, the contrast of the photoresist layer has been enhanced to improve the resolution of the lithographic system. This is accomplished in a well known manner by the use of multilayer photoresists (MLR) of different properties as well as by covering the photoresist with a contrast enhancement layer (CEM). Both of these techniques are not only costly but add complexity to the overall process and result in lower yields due to increased defectivity caused by the more complex processing.
An object of this invention is to improve the resolution of a lithographic process that uses a photoresist by optimizing the contrast of the photoresist.
Another object of this invention is to optimize the contrast of a photoresist without adding to the complexity of the lithographic process.
A further object of this invention is to optimize the contrast of photoresist by controlling the thickness of a layer of photoresist.